I think it WOULD be completely broken if it weren't for hidden card lockouts.. I can't prove it but I'm pretty sure you can't get nature ritual with it, and I've yet to get rejuvenate with it... I don't think I've gotten armaggeddon with it either come to think of it...
We really need to know whats locked out now that we're playing human opponents. If someones playing an illusionist vs illusionist match, and they don't have reflective wall, and they've taken notes on cards for each game they may be able to tell 1/4 of my deck or something.
Imagine
"I don't have reflective wall, so my opponent does."
lets say if he has reflective wall, he can't have geddon, acid rain, nature ritual. (for fear of making uber combos that can be thrown down in 3 turns.)
I have farie sage, elven healer, stone rain, and master healer, sooo
he must have forest sprite (only remaining low cost card).
if he has forest sprite he can't have orc chieften. ** this is a known locout combo
I have wall of fire, dragon, bargul, infero,
so he has fire elemental (only high cost card remaining after geddon) or minotaur chieften (if they went 8-12 from 9-12 like they planned), likely both.
all this before he plays a single card.
And good luck beating him if he's figured out OTHER hidden combos, because even one more fire or earth card eliminated and you could nail down even MORE cards.
What really bothers me is this isn't in any way mentioned in the documentation at all, and more importantly is 100% obfuscated from a new player... heck even some experienced players might not have put together that this is happening yet.
going second may become WAAAY more important as one it lets you see one more opponent card, and two it gives you more time to analyze what cards he wont' have before you have to play something.
EDIT:
Sooo I did some hotseat comaring, and after a dozen hands the list of cards I hadn't gotten with a reflective wall was: Acid rain, nature ritual.
I DID get merfolk apostate, armaggeddon, and reflective wall. so on turn 4 I put out 40something damage, it was cool.